fix: return Value from Function::get_name

[ldm0]

Match V8's Function::GetName return type instead of assuming every function name is a String.

https://v8.github.io/api/head/classv8_1_1Function.html#a145a91e74d5adb78c8b710618253b487

[CLAassistant]

All committers have signed the CLA.

[bartlomieju]

Please add a test that exercises this change

[ldm0]

@bartlomieju test added.